This invention pertains to flywheel energy storage systems and more particularly to vacuum regeneration apparatus and method for a flywheel system that ensures sufficient operating vacuum and has long life with low costs.
Flywheels have been used for many years as energy storage devices. They have often been used as power smoothing mechanisms for internal combustion engines and other kinds of power equipment. More recently, flywheels have been recognized as a very attractive energy storage technology for such electrical applications as uninterruptible power supplies, utility load leveling systems and electric vehicles.
Modem flywheel energy storage systems convert back and forth between the energy stored as rotational inertia in a spinning flywheel, and electrical energy. Such a flywheel energy storage system usually includes a flywheel, a motor generator, a bearing system and a vacuum enclosure. The rotating flywheel stores mechanical energy, the motor generator converts electrical (mechanical) energy to mechanical (electrical) energy and the bearing system physically supports the rotating flywheel.
In almost all energy storage applications, whether quick discharge type (power ride-through), where discharge time is measured in seconds, or long-term discharge type (power backup), where discharge time is measured in hours, flywheels directly compete with electrochemical batteries. Key advantages of flywheels used for electrical energy storage over electrochemical battery systems are its improved longevity and reliability, and its lower long term life cycle cost. Electrochemical batteries, in particular, lead-acid batteries, have short lifetimes, between six months and seven years depending on operating conditions. These batteries require periodic maintenance and can fail unpredictably. In contrast, flywheel energy storage systems are expected to have maintenance-free lifetimes of twenty years. To achieve a maintenance-free life of many years, the vacuum system that is used to prevent excessive drag and aerodynamic heating of the flywheel must be capable of reliably maintaining an adequate level of vacuum.
Accordingly, the invention provides a vacuum regeneration apparatus and method for operating a flywheel system that ensures sufficient operating vacuum and has long life with low costs. The flywheel system is comprised of a flywheel that is supported by a bearing system inside a chamber enclosed within a container. A motor/generator stores and retrieves energy by accelerating and decelerating the flywheel. The vacuum is maintained in the container with a vacuum level sufficient to reduce the aerodynamic drag on the flywheel, typically between 10xe2x88x921 Torr to 10xe2x88x923 Torr depending on the flywheel construction and its operating tip speed.
A vacuum management system, including a vacuum pump and a timer, maintains the vacuum level in the container. The timer periodically enables operation of the vacuum pump, such as a mechanical vacuum pump or a getter pump, to reduce the pressure in the container. In one embodiment, the timer periodically activates a vacuum gauge to measure the internal vacuum; if the internal pressure is above a certain allowable level, a mechanical vacuum pump is started and run for either a set period of time or until the internal pressure falls below a certain level. The periodic activation of the vacuum gauge by the timer, and mechanical pump by the gauge, instead of continuous operation, extends the life of both components and the flywheel system. In another embodiment, the vacuum gauge is eliminated, reducing the cost of the flywheel system and potentially improving reliability, and the timer periodically activates the vacuum pump directly. Understanding of the outgas rates of internal components as well as leak rates allows calculation of the interval when the mechanical pumping is necessary. The pump runs for a short period of time as also controlled by the timer. Because outgassing rates are temperature dependent, calculation of the pumping interval preferably is based on the warmest expected temperatures or on measured temperatures.
Besides use with conventional type mechanical pumps, the vacuum regeneration apparatus and method is also useful for getter vacuum pump flywheel systems. In this case, the vacuum management system includes a getter pump and a timer. Getter pumps can be either evaporable type or nonevaporable type, however use of nonevaporable type getters is preferred because of their both higher sorption capacity per cost and simpler operation. Metal alloy chemical type nonevaporable getters have a large sorbtion capacity for hydrogen, which facilitates long life in flywheel systems, although other types of getter pumps can be employed. In one embodiment, the timer periodically activates the vacuum gauge to measure the internal vacuum and, if the internal pressure is above a certain allowable level, the getter pump is reactivated.
For a nonevaporable metal alloy chemical type getter, reactivation is conducted by heating the getter material to several hundred degrees C, usually for a period of minutes. Hydrogen is normally sorbed to the center of the getter material. However, larger molecules get sorbed only at the surface at room temperature. Reactivation through heating allows the larger molecules to sorb to the center of the getter material, exposing a fresh outer surface. The timer activates power to an electric heater in contact with the getter material that causes reactivation. The timer also preferably turns off the heater after completion of the activation interval. The timer greatly extends the life of the vacuum gauge since it is turned on only periodically, and also controls the getter reactivation interval. In a further embodiment, the vacuum gauge is eliminated. Accurate understanding of the internal outgass rates and outgassing components can allow sufficiently accurate calculation of the rate of getter surface saturation. The timer therefore directly triggers reactivation of the getter periodically to maintain the desired internal pressure. Because the outgass rates will slow over time, the timer can optionally have an interval that also increases over time.